The invention relates to the location of an element of interest contained in a three-dimensional object on the basis of the positions of homologous regions of interest corresponding to the said element of interest and appearing in a set of stereotaxic images of the said object.
Although the invention is applicable to any set of stereotaxic images of an arbitrary three-dimensional object, it is particularly useful in the medical field and, in particular, during a stereotaxic examination in mammography in order to locate microcalcifications in a breast.
A stereotaxic examination in general makes it possible to access, with high precision of the order of one millimeter, a given point of a three-dimensional object, on the basis of two two-dimensional projections of this object acquired, for example, at two opposite angular incidences on either side of the normal to the plane of the image receiver delivering the said projections on the basis of the knowledge, on the one hand, of the position of the projection of this point into the plane of each stereotaxic image obtained and, on the other hand, of the geometry of the stereotaxic imaging apparatus which has led to the construction of the two images, it is possible to calculate the exact spatial position of this point in the three-dimensional object, by trigonometric calculation.
In summary, a stereotaxic examination generally requires:                at least two images of the object at different angulations, and        perfect knowledge of the acquisition geometry of the stereotaxic system, the ability to locate the site of the projection of the chosen element of interest on the various stereotaxic images obtained.        
The first two requirements do not pose a major problem. Currently, stereotaxic examination in mammography is carried out with the aid of a mammograph equipped with a stereotaxic imaging device. The mammograph comprises an X-ray tube located at the end of a first arm which is mobile about an axle and emitting X-radiation to a receiver located at the end of another arm. A breast support plate, or patient support plate, on the one hand and, on the other hand, a compression plate which holds the breast in place during the mammography, are arranged between the tube and the receiver. The image receiver may be a digital receiver, such as a CCD camera, for example, delivering digitized stereotaxic images. The stereotaxic imaging requires the X-ray tube to be rotated around the breast support and compression plates, in two successive opposite orientations on either side of its initial position perpendicular to the plane of the image receiver.
On the basis of the digitized stereotaxic images, obtained, the location of the element of interest on the various images is a difficult operation, in particular in mammography, since it requires corresponding homologous regions of interest in two different stereotaxic images to be set in correspondence or matched with a single element of interest located in the breast.
Thus, in mammography, an attempt is made to match the regions of interest which correspond to the same microcalcification on the various stereotaxic images obtained.
However, the matching of the projected microcalcifications is a difficult problem because the microcalcifications do not look the same from one image to another. Their shape and their contrast may change, as may their arrangement in space. They may be superposed with fibrous zones or with other microcalcifications.
To date, the matching has been carried out entirely manually by the radiologist. The latter would chose those regions of interest which were correspondence on the two angulated stereotaxic images, then enter the two-dimensional coordinates of these regions into a computer which gave the spatial position of the microcalcification, that is to say the three-dimensional coordinates of the latter. On the basis of these coordinates, the radiologist could adjust, for example, the position of a needle holder so that the needle becomes aligned with the microcalcification, in 10 order for it to be punctured, for example.
However, this entirely manual matching proves to be a time-consuming and sometimes inaccurate step.